It is known to those skilled in the art that, under certain condition, a "Karman vortex street" is shed in the wake of bluff cylindrical bodies when a flow of air is perpendicular to the generators of the cylinder. The shedding of vortices occurs periodically first from one side of the body and then from the other in accordance with the velocity of the air flow. Thus, by counting the number of the vortices created by the body in a unit time, the flow rate of the fluid can be measured. One of the vortex shedding flow meters to which the above mentioned theory is practically applied is schematically shown in FIG. 1 which is attached herewith. The flow meter shown generally comprises a vortex shedding body 10 in the form of cylinder with a diametrically extending through hole 12, a sensing means 14 such as electrically heated platinum plate or wire disposed in the moddile of the through hole 12, and a control circuit 16. In operation, the sensing means 14 produces signal representing the numbers of vortices produced in the wake of the body 10 under flowing of the air in the direction of the arrow A. The control circuit 16 evaluates practically the amount of air by computing the signals issued from the sensing means 14 for controlling as a result of this evaluation utilization circuit.
In some of the modern combustion engine driven vehicles, the vortex shedding flow meter of the type mentioned above is used for measuring the amount of air entering the engine to control as a result of this measurement the amount of fuel metered to the engine. The vortex shedding body 10 of the meter is usually set in an inlet tube of an air cleaner of the engine. However, if the inlet tube is not sufficiently long, air flow rate distribution is irregularly changed inevitably as will be understood from FIG. 2, so that the flow rate of the air which flows in the vicinity of the vortex shedding body 10 is frequently changed. Furthermore, when the air flow rate is small and/or the flowing air forms laminar flow in the inlet tube, there appears a considerable difference in air flow rate between a portion near the inner surface of the inlet tube and near the center of the tube. These will cause that accurate air flow measurement is not made by the Karman vortex shedding flow meter.
One of measures for solving the above mentioned problem is to provide the inlet tube with an air flow uniforming device such as a wire netting at a position upstream of the vortex shedding body 10. With this device, a turbulent but considerably uniformed air flow is produced upon flowing of the air in the inlet tube. Experiments have revealed that the presence of the air flow uniforming device permits accurate measurement of the flow meter especially when the flow rate is low.
Thus, such induction apparatus as just mentioned can perform accurate control of air-fuel ratio of the air-fuel mixture when the engine is under low speed operation, such as, under idling condition. However, when a high speed operation of the engine where a big volume of air is necessary is required, the presence of the flow uniforming member causes a trouble in that a considerable resistance of air flow is produced causing drop of output power of the engine.